Gastric inhibitory polypeptide that is also referred to as a glucose-dependent insulinotrophic polypeptide (hereinafter abbreviated as GIP) is one of gastrointestinal hormones, and is secreted during eating from K-cells that are present in the small intestine. It is known that GIP has an effect of suppressing secretion of gastric acid and an effect of suppressing gastric motility (see Non-patent Literatures 1 to 3).
It is also known that GIP enhances secretion of insulin from pancreatic β-cells and accelerates uptaking of glucose into adipose cells in the presence of insulin. Therefore, it is considered that the effects of GIP are contributory to obesity, and in fact, it was reported that obesity is suppressed when the function of GIP is inhibited (see Non-patent Literature 4).
Furthermore, it was reported that GIP is contributory to insulin resistance (see Non-patent Literature 4). When insulin resistance is developed, an effect of absorbing sugars by insulin decreases, which results in occurrence of hyperinsulinemia. It is also said that hyperinsulinemia is an underlying cause that leads to development of various lifestyle diseases such as obesity, and thus it is important to prevent or improve insulin resistance in view of reduction of risks of lifestyle diseases.
In view of the above, to effectively suppress GIP is expected to lead to effects such as enhancement of digestion, improvement of slow digestion, and prevention or improvement of obesity and insulin resistance.
By the past studies, 3-bromo-5-methyl-2-phenylpyrazolo[1,5-a]pyrimidin-7-ol (BMPP) and pyrazolopyrimidine compounds have been known as substances that inhibit the functions of GIP. Furthermore, guar gum and the like have been known as substances that suppress secretion of GIP after eating (see Patent Literatures 1 and 2, and Non-patent Literatures 5 to 10). However, these substances cannot be considered to be sufficient from the viewpoints of safeness and effects.
Meanwhile, polyglutamic acids are widely used as moisturizing agents, absorbing agents and the like due to their high water retaining ability in the fields of foods, medical treatments, cosmetics and the like, and gain attentions as highly safe biodegradable polymers. Furthermore, it was reported that polyglutamic acids have an effect of promoting absorption of calcium from the small intestine and an effect of suppressing elevation of blood pressure (for example, see Patent Literatures 3 and 4). Furthermore, it was suggeated an agent for improving a blood sugar level using a polyglutamic acid, in order to suppress elevation of blood sugar level (see Patent Literature 5).